Indeed I do,
Alexanndra. But I don't think it's too severe. At least I hope I can help to shed some light on the confusion.
Perhaps one big part of the problem is the use of terminology. What do the textbooks mean when they say "prediction"? Does it always mean that the speaker is looking into future time to suppose something 'will happen'? Or could we reword it a bit to say that a prediction occurs when a speaker feels he is in possession of certain knowledge that leads him to believe that some particular thing or event
is true, but at the same time, because of the circumstances pertaining at the moment, has no possibility of being absolutely certain about it. I know that is a bit of a mouthful, and sort of roundabout, but it allows for such expressions as:
"I'm sure they'll be asleep by now." (not looking into the future)
On the other hand, a speaker may believe his present knowledge constitutes
evidence that a consequence is already underway in some important respect. His "prediction", then, about that result, is merely what he sees as the inevitable outcome of events
already in progress. In other words, he is just looking ahead to point out what will surely happen unless something intervenes to stop the events already underway.
Sometimes, it may be possible in particular speaking situations for a speaker to look at something in either way. The choice is up to him, hence you may have occasions where a speaker could either say "will" OR he could look at it differently and say "be going to". Those are likely to be rare occasions, however, because if you look at a large number of occasions where "prediction" is involved, most will fall naturally one way or the other. The opposite point-of-view for the speaker would seem strange. An example of this is: "It's going to rain." In the circumstances where a person might say this, given the occurance of dark clouds in the distance, it would be extremely unlikely that he would view this as fitting the situation where "will" would be appropriate. He is not likely to say: "It will rain."
OK, let's now look at the sentences and situations you have proposed. In the case of "I'm going to be sick if I don't get an 'A'." it seems to me that this speaker, since she is speaking about herself, is in full control of whether to look at it as something that is probably true, but she can't know for sure since she doesn't yet know if she has the 'A', or whether she is actually "planning" to be sick...it's just a matter of time. So she, in this case, can say it either way:
"I'll just be sick if......." (she thinks so, but she's not sure)
"I'm going to be sick if...." (she "knows" what happens when she gets poor grades, and she's just waiting for it to happen this time)
"Why don't you ask Larry, he'll help you." is pretty clear, as you say. This speaker has no way to assume that Larry's help is already coming. She may assume that Larry will help, on the basis of past experience with Larry, but she really has no way to know for sure. It fits the use of "will".
The same clarity (as you point out) is available in the case of, "...I'm going to paint my room." In this case, her knowledge of her own plan (particularly because she views it as already underway, since she has already purchased the paint) makes it reasonable for her to view 'painting her room' as an inevitable outcome. Something that she feels is sure to happen.
"Sue will graduate in June."
"Sue is going to graduate in June."
I believe that a speaker looking at this situation could see it in either of two ways: (1) when June arrives, Sue 'probably' graduates (but then, who knows?), or (2) Sue is already on track to graduate. She has completed all her required classes, and is getting good grades in the last group of classes she's taking now, so graduation appears to be a sure thing.
Does this long, involved 'explanation' help you at all,
Alexanndra? I hope it does. It seems clear to me now, but I'll admit it took some time for me to absorb the distinction between 'inevitable outcome' and 'probable, but unknowable outcome'. Good luck with your students. They sound like a great class because it sounds as if they are really "involved" with your teaching. That's wonderful.
Larry Latham